In subject area: Biochemistry, Genetics and Molecular Biology
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2022, Food Bioscience Tolulope Joshua Ashaolu, ... Ibrahim Khalifa
Peptides are specific protein fragments with supposedly improved activity and physiological functions. Peptides are of many types with dipeptides being the shortest of them all because they consist of just two amino acids linked by a peptide bond. Conversely, polypeptides usually contain >50 amino acids. These basic descriptions conveniently allow peptides to be called biological polymers.
Details on the identification of peptides are discussed in the next section. For their classification, they are based on numerous factors. For instance, if they are to be classified based on production, there are the peptones, produced from the breakdown of animal milk/meat, or built up from metals, fats, vitamins, and certain salts (Payne, 1976). In cultures and microbiology experiments, the peptones are used in preparing nutrient media, particularly for bacterial and fungal growth. Another class in this category is the milk peptides. They are formed from casein during milk protein digestion in the gastrointestinal tract or lactic acid bacterial fermentation of milk. Ribosomal peptides are produced by cellular ribosomes in a translation process of ribonucleic acid (RNA) into messenger RNA (mRNA), which is an AA sequence. The residues are linked together to form more matured peptides. Some bacteria produce microcins or bacteriocins, which are peptides, as their substances of defense mechanism, but these peptides undergo post-translational modifications such as hydroxylation, phosphorylation, sulfonation, and glycosylation (Duquesne, 2007). Another class of peptides based on source include the non-ribosomal peptides, which are formed by enzymes that are not ribosomes, such as glutathione (Anderson & Meister, 1996, pp. 213–226). They also undergo the same post-translational modifications as ribosomal peptides after the amino acid sequences are connected. Several peptides are linear, but some are cyclic, forming rings and/or are lariat in structure (Pons et al., 1991). Non-ribosomal peptides often are cyclic, and therefore, have highly complex structures, but less often, D-amino acids are produced from the racemization of L-amino acids occurring within peptides (Torres et al., 2002). Lastly, under this category, there are peptide fragments, which are protein fragments used to identify protein sources (Hummel et al., 2007). Other than being controlled in the laboratory, the natural degradation of paleontological or forensic samples can also form peptide fragments (Webster & Oxley, 2005).
Based on function, there are neuropeptides, which are associated with neural tissues. Lipopeptides have lipid connections, and some of them have interactions with G-protein coupled receptors (GPCRs); they are called pepducins. Hormonal peptides act as hormones, while proteoses are unpurified peptides after protein hydrolyzation. Then, based on family, there are antimicrobial peptides (e.g. magainin, cecropin, cathelicidin, and defensin); tachykinin peptides (e.g. kassinin, neurokinin, and eledoisin); vasoactive intestinal peptides (e.g. secretin and glucagon); pancreatic polypeptide-related peptides (e.g. neuropeptide Y, peptide YY, and pancreatic polypeptide); opioid peptides (e.g. proopiomelanocortin, enkephalin pentapeptides, and prodynorphin); calcitonin peptides (e.g. calcitonin and amylin); and self-assembling peptides (e.g. aromatic short peptides, biomimetic, peptide amphiphiles and, peptide dendrimers) (Hamley, 2011; Zhu et al., 2018; Kastin & 2nded., 2013). Whether based on function, source or family classes, other peptides include antimicrobial, anticancer, cardiovascular, endocrine, fungal, gastrointestinal, invertebrate, opiate, plant, renal, respiratory, and venom peptides (Khalifa et al., 2019).
Structurally, linear peptides have free NH 2 and COOH at both respective ends while branched peptides have many amino acids branching out on a linear peptide chain via either the ω-carboxylic group of dicarboxylic amino acid, or the ε-amino group of lysine. Cyclic peptides do not have N-terminal or C-terminal end. Semi-cyclic peptides have just one end, in which if it is an N-terminal, the α-COOH of the last amino acid will be linked to the ε-NH 2 of an endopeptide lysine. If it is a C-terminal, the α-NH 2 of the first amino acid will be linked to the ω-COOH of an endopeptide dicarboxylic amino acid (Du & Shen, 2001; Torres et al., 2002).
2013, Handbook of Biologically Active Peptides (Second Edition) Johji Kato, Kazuo Kitamura
In 1993, a biologically active peptide was isolated from the acid extract of human pheochromocytoma tissue by monitoring of the elevation of cAMP levels in rat platelets.13 This bioactive peptide was named “adrenomedullin” because it was found to be abundantly present in the human adrenal medulla and in pheochromocytoma.13 Human adrenomedullin (AM) consists of 52 amino acids with a ring structure formed by a disulfide bond and an amidated tyrosine at the C-terminal. The amino acid sequence of AM resembles those of calcitonin-gene-related peptide (CGRP) and amylin, belonging to the CGRP superfamily (Fig. 1). Sequence homology among these bioactive peptides is relatively low, but they share two molecular structures: a disulfide bond and an amidated C-terminal, both of which are essential for exerting biological actions.3,10,12 In 2004, two independent research groups discovered another member peptide of the CGRP superfamily by searching the genomes of humans and other vertebrates.31,34 The two groups named this 47-amino-acid peptide adrenomedullin-2 (AM2) and intermedin, respectively. As shown in Table 1, AM has a wide range of actions on the cardiovascular system, as described later in this chapter. The biological property of AM2/intermedin has also been examined since this discovery. According to the findings so far reported, the biological effects of this family peptide are mostly similar to those of AM, while some differences in potency or receptor selectivity have been detected, raising the possibility for a receptor or intracellular signaling system differing from those of AM.5 AM2/intermedin is further discussed in the Renal Peptides section of this book.
FIGURE 1. Amino-acid sequences of human adrenomedullin (AM), calcitonin-gene-related peptide (CGRP), amylin, and AM2/intermedin. Asterisks indicate amino acids identical to those of AM.
